The present invention relates to electrical apparatus of the kind having an electrode having an area exposed to current flow through a conductive medium, e.g. a conductive liquid, which may or may not contain solids. Such a conductive medium will be referred to herein as an "electrolyte" for convenience irrespective of the mode of conduction in the liquid and of whether any electrolysis occurs. Such an arrangement is found in electrical apparatus intended for many different purposes, for instance ohmic heating apparatus, electrolysis apparatus and electrical batteries.
The performance of such apparatus is often limited by the maximum current density which the electrode can sustain. It is often found that the current density is not constant over the whole of the surface area of the electrode exposed for current flow but tends to increase substantially at certain parts of the electrode, typically at the edges of the electrode. This may be because additional current paths are provided by a volume of electrolyte situated laterally beyond the edges of the electrode or may be due to the shape of the electrode. In order to keep the maximum current density to which any part of the electrode is subjected down to an acceptable level, the bulk of the surface area of the electrode has to be operated at a current density which is considerably less than would be preferred.
In some ohmic heating cases it may be possible to avoid this problem by increasing voltage rather than current so that the desired heating power can be obtained at a permissible current density. However, increasing voltage may cause problems such as operator safety, or damage by arcing. Unless means for limiting the current are included higher voltages will themselves lead to higher currents passing in a given system. The use of current limiting factors may in itself cause problems.
It may be possible to increase electrode size but this may cause problems. Electrode materials are frequently expensive, e.g. incorporating precious metal coatings and in some forms of apparatus size may be inherently undesirable. For instance, in ohmic heaters used for heating foodstuffs, the use of a large electrode area implies the use of large heating chambers. This in turn makes rapid heating more difficult. Slow heating is undesirable because of the consequent loss of flavour, vitamins, texture or other factors affecting quality through long exposure of foodstuffs to heat.
It would be desirable therefore to provide some means for limiting the increase of current density at particular areas, e.g. towards the edges, of the electrode in such apparatus.
It has been proposed previously, e.g. in GG-A-526238 and U.S. Pat. No. 2,584,654, to vary the proportion of the area of an electrode immersed in an electrolyte which is effective in passing current by positioning a moveable shield member between the electrode and a counter electrode. Movement of the shield member to expose a varying amount of the electrode to current flow may be used to control the gross current flow through the apparatus. The aim of the shield member in such systems is not to control current density at particular locations on the electrode but simply to limit the overall current flow so that with the shield member in place, the apparatus operates substantially below its maximum current capability.